Conventionally, a technique has been increasing in which a treatment is performed by frequently replacing various treatment instruments with a guide wire employed as a guide, when a disease in a pancreaticobiliary duct system is endoscopically treated. Therefore, as disclosed in Patent Document 1, for example, an endoscope has been developed in which a guide wire can be fixed to the distal end portion of the insertion unit of the endoscope in order to perform the replacing operation of the treatment instruments in a short period.
An internal diameter of a lumen of a duodenum in a living body is generally about 30 mm. A thickness of a side-view type endoscope for a duodenum is generally set to about 10 mm. Therefore, a maximum of the horizontal distance from a papillary opening portion to an observation window of the endoscope is about 20 mm. On the other hand, the papillary opening portion is anatomically positioned at the lower part (at the side of a small intestine) of the papilla. The advancing direction of a bile duct from the papillary opening portion extends in the upward direction from the papilla (toward a stomach).
Therefore, a viewing direction of an observation optical system in the side-view type endoscope for a duodenum is set so as to be inclined rearward (toward the stomach) at an angle of about 5 to 15 degrees beforehand. The endoscope is designed such that, when the papillary opening portion is observed from the front (at the position at the center of the field of view), the distal end portion of the endoscope is located at the position where the papilla is looked up and the observation and treatment are inevitably done easily. Additionally, when the papillary opening portion is caught at the center of the field of view of the endoscopic image, the endoscopic image of the papillary opening portion that is sharp and bright, and has less distortion can be obtained, compared to the case where the papillary opening portion is caught at the peripheral portion of the screen. Therefore, from the viewpoint of facilitating the replacing operation of the treatment instrument afterwards, it is important for an operator of the endoscope to operate the position of the distal end portion of the endoscope so as to position the distal end portion of the endoscope at the center of the field of view or at the position slightly above the center of the field of view for increasing a sense of looking up the papillary opening portion. In other words, this position is the basic position of the endoscope for a duodenum.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-34905